This invention relates to systems which permit detecting and signalling "hard" landings of aircraft, landings which due to a violent impact, may cause immediate damage to the airplane, or landings of lesser intensity which if repeated, may damage the structure of the plane resulting in breaks in the landing gear or of the air frame on the ground or in flight.
Devices for detecting abnormal landings have been previously proposed. These devices are based on various principles.
There is a known device which determines the depression of the shock absorbers of the main landing gears on an airplane consisting of for example an indicator of lead which will be marked by the arrival of the shock absorber at the end of its stroke.
This system is not reliable, since it is known that it is possible to effect a hard landing without the shock absorber reaching the end of its stroke. This can occur upon the takeoff of the airplane with the system indicating an erroneous measurement.
A second known device makes it possible to note an excessive increase in pressure in the shock absorbers of the landing gear. The pressure of the hydraulic liquid of the shock absorber is a function of the internal force within the shock absorber, which is related to the vertical force which the airplane experiences.
This system requires substantial changes to the shock absorber making this device expensive. Furthermore, this system is frequently inaccessible, particularly when the shock absorbers are integrated in the landing gear leg.
Another known device employs a piezoelectric accelerometer which becomes active only when the landing gear is extended.
This device has the drawback of detecting all accelerations which occur when the landing gear is out, whatever the cause thereof, which is very disturbing in the case of combat airplanes which experience substantial vertical acceleration in the "landing gear out" configuration which the detector will indicate in the same manner as those coming from a hard landing.
Finally, it has been suggested to use a purely physiological detection on the part of the pilot, who feels a strong acceleration and can report it.
This last-mentioned solution is ineffective since the impact at the time of the landing is very brief and furthermore it is filtered by the seat and by the body of the pilot, who is therefore incapable of quantifying the received acceleration.